1. Field of the Invention
The present invention relates to a high aperture ratio liquid crystal display (LCD) device. More particularly, the present invention relates to an in-plane switching (IPS) mode LCD capable of enhancing an aperture ratio by forming a storage capacitor, namely, a light blocking region, to overlap with a lower end of at least one electrode.
2. Description of the Related Art
As the demand for various information display devices is increasing, research for a flat panel display device such as an LCD (Liquid Crystal Display), PDP (Plasma Display Panel), an ELD (Electro Luminescent Display), an FED (Field Emission Display) or a VFD (Vacuum Fluorescent Display) is actively ongoing. Of these displays, the LCD is the most spotlighted because of its implementation of high picture quality, mass-production techniques, ease of a driving unit, light weight and thinness, and low power consumption.
The LCD device is a display device for displaying a desired image by controlling a light transmittance of a liquid crystal layer by pixels arranged in a matrix form by separately applying a data signal according to picture information of each pixel. The LCD device is commonly driven by an AM (Active Matrix) method. The AM method is a method in which a switching device such as a thin film transistor (TFT) is added in each pixel to apply a voltage to liquid crystal of each pixel in order to drive the liquid crystal.
The LCD device can be divided into various types of display mode LCD device based on how the liquid crystal molecules are driven. Among the various types of display modes, a TN (Twisted Nematic) mode LCD device has been commonly used.
The TN mode LCD device, turns on/off an electric field perpendicular to a substrate to drive liquid crystal molecules so that a director of liquid crystal can have an angle between 0° and 90° to a substrate. The TN mode LCD device has advantages of an easy black and white display function, a high speed response and a low driving voltage.
However, since the TN mode LCD device drives liquid crystal molecules to be vertical to the substrate, its viewing angle characteristics are not good. In particular, it has viewing angle dependence such that a screen color or brightness of an image changes based on a direction or an angle in or at which a user looks at a screen of the LCD device. Thus, in an effort to overcome such shortcomings, an in-plane switching (IPS) mode LCD device for driving liquid crystal to be driven in a state that a director is horizontal to the substrate is being actively studied.
In the IPS mode LCD device, when a voltage is applied to an electrode, an in-plane electric field, namely, a field having a direction parallel to the substrate, is formed to align liquid crystal molecules horizontal to the substrate to obtain wide viewing angle characteristics compared with an existing viewing angle. FIG. 1 is a schematic view of the IPS mode LCD device.
As shown, on an array substrate of the IPS mode LCD device, a gate line 1 and a data line 3 are arranged horizontally and vertically, respectively, to define a pixel region. In the pixel region, at least a pair of common electrodes 6a and 6b for switching liquid crystal molecules and a pixel electrode 7 are disposed in an alternating pattern substantially parallel to the data line 3 to generate an In-plane electric field on the array substrate.
A switching device, namely, a thin film transistor (TFT 9), which includes a gate electrode 1a, a semiconductor layer (not shown) and source and drain electrodes 2a and 2b is formed at the crossing of the gate line 1 and the data line 3, transfers a data signal to the pixel electrode 7 to switch liquid crystal molecules. The pixel electrode 7 is electrically connected with the storage electrode 13 extendedly formed from the drain electrode 2b through a first contact hole 11 to receive a signal. Also, in the pixel region, a common line 4 is formed parallel with the gate line 1 to transfer a common signal to the common electrodes 6a and 6b. 
The storage electrode 13 extendedly formed from the drain electrode 2b overlaps with the common line 4 with a gate insulation film (not shown) interposed therebetween, to form a storage capacitor.
As a result, in the IPS mode LCD device having the above-described structure, the common electrodes 6a and 6b and the pixel electrode 7 generate an in-plane electric field on the array substrate to drive liquid crystal molecules in a state of being parallel with the substrate, so that the viewing angle in the up/down and left/right direction can be considerably enhanced when the display unit of the LCD device is viewed from the front side, compared with the related art TN mode LCD device.
In the IPS mode LCD device, because the common electrodes 6a and 6b and the pixel electrode 7 are disposed on the same substrate, the aperture ratio and luminance are reduced compared with the related art TN mode LCD device.
The aperture ratio is determined by a region where light of a backlight unit made incident on the liquid crystal panel cannot pass through the liquid crystal panel, namely, an area of a light blocking region. In the related art TN mode LCD device, the light blocking region is determined by a line part of the gate line and the data line applying a signal voltage, the TFT part, an extra part for considering an alignment error in bonding upper and lower substrates of the liquid crystal panel, and the part of the storage capacitor. Comparatively, in the IPS mode LCD device, a part of the common electrodes and the pixel electrode is added as the light blocking region, which reduces the aperture ratio and luminance of the LCD device.
Although the common electrodes and the pixel electrodes are formed as transparent conductors in terms of increasing white luminance of the IPS mode LCD device, in-plane electric field is substantially generated at the edge of electrodes, so the central region of the electrodes does not help to increase the aperture ratio.